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Computed Tomography. Basic principles. V.G.Wimalasena Principal School of Radiography. Introduction. Computed tomography (CT) is a medical imaging method employing tomography . The word "tomography" is derived from the Greek tomos (slice) and graphein (to write).

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Computed tomography

Computed Tomography

Basic principles



School of Radiography

Computed tomography


  • Computed tomography (CT) is a medical imaging method employing tomography.

  • The word "tomography" is derived from the Greektomos (slice) and graphein (to write).

  • A large series of two-dimensional X-ray images (slices) of the inside of an object are taken around a single axis of rotation.

  • Digital geometry processing is used to generate three-dimensional images of the object from those slices.

Computed tomography


  • The first commercially viable CT scanner was invented by Sir Godfrey Hounsfield in Hayes, United Kingdom at EMI Central Research Laboratories using X-rays. Hounsfield conceived his idea in 1967. and it was publicly announced in 1972.

  • Allan McLeod Cormack of Tufts University in Massachusetts independently invented a similar process, and both Hounsfield and Cormack shared the 1979 Nobel Prize in Medicine.

Computed tomography


  • gantry aperture (720mm diameter)

  • microphone

  • sagittal laser alignment light

  • patient guide lights

  • x-ray exposure indicator light

  • emergency stop buttons

  • gantry control panels

  • external laser alignment lights

  • patient couch

  • ECG gating monitor

Computed tomography


  • x-ray tube

  • filters, collimator, and reference detector

  • internal projector

  • x-ray tube heat exchanger (oil cooler)

  • high voltage generator (0-75kV)

  • direct drive gantry motor

  • rotation control unit

  • data acquisition system (DAS)

  • detectors

  • slip rings

Computed tomography

Understanding Basic factors

  • Absorption :-stopping of x-rays with transfer of energy

  • Scatter:- deflection of x-rays

  • Incident Intensity :- No. of x-ray photons falling on an object

  • Transmitted Intensity:- No. of photons passing through

Scattered x-rays

Transmitted X-ray beam

Incident x-ray beam

Computed tomography


The reduction of the beam intensity on passing through the material due to absorption plus scatter

The degree of attenuation is obtained by measuring and comparing the incident and transmitted intensities

More dense material

Less transmitted x-rays

More transmitted x-rays

Less dense material

Computed tomography

Applications of X-ray attenuation & detection

  • Conventional X-ray (Radiography)

  • Conventional Tomography

  • Computed Tomography

Computed tomography

Conventional X-Ray

  • Conventional x-ray produces a compression of a volume to a plane

  • The detector is the Silver halide crystal on a x-ray film

  • The degree of blackening represents the total attenuation through the path of x-ray photons

Computed tomography

  • The higher the attenuation the lesser is the blackness

  • The structurewhich results more attenuation or more transmission predominates in the image

Computed tomography

Conventional Tomography

  • The source and detector moves

  • Produces Images of coronal or sagittal sections (cuts) of areas of interest

  • Eliminates the superimposition of structures above and below

Computed tomography

CT Scan

  • CT scan produces axial sections/cuts /Slices

  • The CT image is recorded through a SCAN.

  • Scan?

  • A scan is made up of multiple X-Ray attenuation measurements around an objects periphery

X-ray tube


Computed tomography

Slice / Cut

  • The cross sectionalportion of the body which is scanned for the production of CT image is called a slice.

  • The slice has width and therefore volume.

  • The width is determined by the width of the x-ray beam

    To be continued ….CTComplementary2